Response queue poisoning is a desynchronization flaw in which an HTTP response is matched to the wrong request or client. It usually happens in systems that reuse backend connections or pass traffic through multiple intermediaries, such as reverse proxies, CDNs, and load balancers. If those components disagree about message boundaries, one client’s request can shift the response queue and cause the next response to be delivered to the wrong session.
This matters because it can leak data across users, expose cookies or tokens, and create hard-to-trace application bugs. Attackers often reach it through request smuggling or other parser-differential issues that corrupt the order of requests and responses. Defenders look for inconsistent parsing, unexpected 404s, and out-of-order replies, then reduce connection reuse, align HTTP handling across layers, and harden cookies with HttpOnly, Secure, and SameSite to limit damage if a response is misdelivered.